1.3 Stars, Galaxies & the Universe

Star

Massive ball of plasma held together by gravity; generates energy through nuclear fusion.

Nuclear Fusion

In stars: hydrogen nuclei fuse into helium, releasing enormous energy (E = mc²). Powers all stars.

H-R Diagram

Plots stars by temperature (x-axis, hot on LEFT) vs. luminosity (y-axis). 90% of stars on 'main sequence.' Sun is middle main sequence.

Supernova

Explosive death of a massive star (≥8× Sun's mass). Produces elements heavier than iron. Leaves neutron star or black hole.

Black Hole

Region of space where gravity is so intense nothing escapes (not even light). Event horizon = boundary.

Nebula

Cloud of gas and dust; birthplace of new stars (e.g., Orion Nebula).

Galaxy

Massive collection of stars, gas, dust, dark matter. Types: spiral, elliptical, irregular.

Milky Way

Our galaxy; barred spiral; ~100,000 light-years across; Sun is in outer Orion Arm.

Big Bang Theory

Universe began ~13.8 billion years ago from a single hot, dense point. Supported by: (1) expanding universe/redshift, (2) cosmic microwave background radiation (CMB).

Red Shift

Light from galaxies stretches toward red wavelengths → galaxies are moving away → universe is expanding.

Solar Nebula Theory

Planets formed from a spinning disk of gas and dust ~4.6 billion years ago. Explains why planets orbit in same plane.